How to generate spherical gradient volume
Volumetric Emmision


add Emmision shader plugged into volume socket of material Output
add Transparent
add Mix shader - with Emmision up 
add Gradient texture > Spherical as factor


Volumetric Absorption

Instead of Emmision use Volume Absorption node.

3D Illusion

In some cases it's quite efficient to fake it by simple circle object with Spherical Gradient texture assigned to alpha. Circle orientation Tracked to Camera, so moving object is always facing to camera that make in illusion of 3d object.

(In case you don't care about shadows.)